The impact of detritus accumulation on the performance of siphonic rainwater outlets

Abstract

This paper presents the findings of an 18-month study of detritus accumulation at two siphonic rainwater drainage systems installed in a building in Edinburgh. Findings are based on an analysis of data recorded from the site, and are positioned within the context of enhancing representation of the outlet loss coefficient used within both steady-state design calculations and finite-difference-based unsteady flow modelling techniques. The scope of findings reported herein is extensive but, in the main, shows how detritus accumulation not only builds relatively rapidly but also that ‘wash-through’ or ‘displacement’ occurs. Further, data suggest that detritus accumulation in the gutter, that is either wind-blown or is introduced from roof runoff, can, and does, build around the outlet with only relatively low intensity rainfall but that high-intensity rainfall events do seem to result in a direct and significant increase in blockage. An indication of the impact upon performance, assessed using simulation software, is also presented. Practical application: This paper reports patterns of detritus accumulation at on-site siphonic rainwater outlets. Changes in detritus ‘categories’ are mapped to corresponding weather data and conclusions drawn on influencing factors. The paper therefore yields key information for building owners/operators on the extent of detritus accumulation at siphonic system outlets. Outcomes also inform designers of the potential shift in loss coefficient and flow performance post-installation.